Internal fixation devices, such as those used in craniomaxillofacial surgery historically have been made of various materials including metals such as titanium. Poly(lactic acid) or polylactide polymers have also been successfully utilized as medical implants due to their biocompatibility with biological tissues; degradability in vitro and in vivo; and good mechanical properties. Extensive work has been carried out by several investigators in understanding the morphological properties of poly(L-lactic acid) [PLLA]. In particular, considerable progress has been made in elucidating the crystalline structure (4) and crystallization kinetics of PLLA. Recently, detailed studies were carried out to investigate the influence of physical aging on the viscoelastic behavior of PLLA, and the effects of water sorption on the internal motions in PLLA and other related polymers. The influence of morphology (crystalline and amorphous) on the degradation of PLLA was conducted in aqueous media for periods up to 2 years. It was determined from this study that the highly crystalline residues appear to be very resistant to degradation, and that degradation proceeds more rapidly in the center than at the surface for both the crystalline and the amorphous specimens. (S. Li and S. McCarthy, Biomaterials, 20, 35, 1999. H. Cai, V. Dave, R. A. Gross, S. McCarthy, J. Polym. Sci., Polymer Physics, 40, pgs. 2701-2708, (1996). S. Li, H. Garreau and M. Vert, J. Mater. Sci.: Mater. Med., 1(4), 198, 1990).)
Recently, internal fixation devices fabricated from biodegradable polymers such as poly(lactic-co-glycolide) (PLGA) have become popular. Fixation devices made of these types of materials have advantages over older metallic devices: they do not corrode; they can be constructed in such a way as to avoid stress yielding; and they are resorbable which obviates the need to remove the devices. Further, these devices are specifically designed for use in the pediatric patient population as their resorption eliminates any adverse, restrictive effect that permanent plates would impose on craniomaxillofacial growth and development.
Craniofacial surgery is performed routinely in the United States and around the world for numerous problems involving the skull. These include craniosynostosis (premature fusion of the cranial sutures); skull deformities associated with syndromes such as Crouzon Syndrome and Apert Syndrome; skull deformities resulting from the resection of both benign and malignant tumors; and complex craniofacial trauma involving the bones of the face and skull.
Resorbable plates and screws are, for example, routinely utilized in the pediatric population for the stabilization of bones during reconstruction in each of these scenarios. The use of screws to secure plates requires additional cumbersome power equipment that necessitates additional operating room staff training and cost as well as additional surgical time that increases the cost of the operating room, anesthesia time and surgical time. A product that can eliminate the need for screws but still permit satisfactory bony stabilization for craniofacial reconstruction would yield a great medical advance in the field of craniofacial surgery and pediatric care by (1) simplifying and expediting the intra-operative application of plates to the skull, and (2) making power equipment for drilling holes for the use of screws entirely unnecessary.
At present, several types of craniofacial surgery plating systems are currently commercially available. Those made by Stryker-Leibinger and Synthes include titanium systems as well as resorbable polymer-based systems. The resorbable systems require fixation with resorbable screws. Based on the polymers used in these systems, resorption of plates and screws occurs approximately 2 years following placement. A new product produced by KLS Martin is the only internal fixation product that does not use screws, per se, for fixation. The product, Sonic Weld,™ instead of using screws requires that a tack be applied directly into a drilled hole. An ultrasonic device then melts the resorbable tack within the hole. Thus, no actual screwing takes place but the material is melted into the hole and secures the plate in that fashion. Equipment is still required to facilitate drilling of these holes for placement of these tacks. This method has been criticized for the unknown effect of the material permeating the bony trebeculae. Furthermore, such a method still requires the use of power equipment during surgery.
Further, an internal fixation system that contributes to the quality of bone healing by the administration of growth factors or other biologically-active molecules, would be an invaluable addition to the armamentarium of the reconstructive craniofacial surgeon. The invention described herein can be impregnated with such biologically-active (bioactive) molecules due to the nature of the co-continuous polymers utilized which permit introduction of pores into the actual plate structure, yielding a porous, bioactive plate. Further, the size of these pores, and hence the degree of porosity, can be selectively controlled to permit molecules of varying sizes to be impregnated into the structure of these plates. As such, the biodegradable, resorbable bone plates described in this invention are the first such porous plates to be utilized as craniomaxillofacial bone plates.
Introduction of pores to the resorbable plating system described in this invention permits more rapid resorption of the plates. As bone healing occurs fully within 6 weeks following bone fixation during reconstructive craniofacial surgery for the management of either congenital deformities or fractures, fixation systems are not required beyond this time point. Plate porosity permits controlled plate resorption within 3-6 months following placement, considerably earlier than other resorbable plating systems.